Step transducer between over-moded circular waveguides

The amplitudes of the spurious modes due to a small step in an over-moded circular waveguide are computed, assuming that one of the waveguides is fed by a pure H01 mode. It is shown that the amplitude of the H02 mode is the largest of the spurious modes. For the reduction of the conversion losses, a simple transducer is designed. Its radius is the geometrical mean of the radii of the two waveguides to be connected. Its length is one-half beat-wavelength of the H01 and H02 modes.